Biometrics is used for identity recognition, which is based on individual unique biometric information such as fingerprint, face, veins, iris, and retinas. Fingerprint recognition has been extensively used and also become a useful tool for criminal investigation, but fingerprints are relatively easy to duplicate. Vein recognition relies on vein features extracted from the vein distribution in human hands, and thus is advantageous for providing high accuracy and reliability because individual vein distribution is unique and not easily forged. The underlying principle thereof is that the deoxygenated hemoglobin in veins absorbs infrared light and thus veins will be seen as dark lines in an image taken from human hands under projection of infrared light. Recognition then can be achieved according to vein features such as the pattern, distribution, width, color, etc. However, vein recognition is significantly affected by physical conditions of human body. For example, in cold days, veins may contract and become too thin to be sampled, and venous diseases may also cause vein recognition to be impossible. Face recognition relies on facial features such as the facial contour and relative positions of the five sense organs. This approach is convenient because an ordinary camera can be used to capture the image of a face. However, the captured sample for comparison tends to be interfered by facial expressions, ambient light, hair styles and so on, and is not distinguished between twins, and thus the accuracy of face recognition is an issue to be specially considered.
Biometrics has been extensively used in many applications such as information, communications, and security, for identity recognition. The development of biometrics has been made toward improvements in comparative performance and tolerance, which are typically measured by false acceptance rate (FAR) and false rejection rate (FRR). FAR is referred to the probability that an unauthorized user is accepted as an authorized user, and FRR is referred to the probability that an authorized user is mistaken for an unauthorized user and rejected. To any biometric system, there is always a tradeoff between comparative performance and tolerance. If tolerance is increased for convenience of authorized users (low FRR), an unauthorized user can pass examination easier (high FAR). If tolerance is decreased for preventing unauthorized users from access (high FAR), it is difficult for authorized users to pass examination (low FRR). Therefore, existing biometric systems are less flexible and very hard to balance operational convenience and high recognition rate. The false rate remains high no matter the comparative criteria are set strict or loose.